Ethylene and propylene are the essential materials used in modern chemical engineering. In chemical industry, ethylene and propylene, as mainly originated from light oils (naphtha and light diesel) through processing of petroleum, fully rely on petroleum resources. Along with the economic development, more-and-more alkenes are required. Therefore, developing a new technology for alkene production has always been an important research direction. Thereinto, the methanol to olefins process has been regarded as the most successful non-petrochemical route for producing ethylene and propylene from coal resource. For the past decades, a huge amount of both human and material resources have been devoted to the research of the process.
In 1976, the Mobil Oil Corporation, for the first time conducted conversion reaction of methanol into hydrocarbons on a ZSM-5 molecular sieve catalyst. After that, the conversion from methanol to gasoline on a ZSM-5 molecular sieve catalyst was disclosed in U.S. Pat. No. 4,035,430; the technology of producing low-carbon alkenes from methanol on a ZSM-5 molecular sieve catalyst was disclosed in U.S. Pat. No. 4,542,252; and the process for producing low-carbon alkenes from methanol on a ZSM-5 molecular sieve catalyst modified by phosphorus, magnesium, silicon or alkali metal elements was disclosed in U.S. Pat. No. 3,911,041, U.S. Pat. No. 4,049,573, U.S. Pat. No. 4,100,219, JP 60-126233, JP 61-97231, JP 62-70324 and EP 6501.
Additionally, the applicant also disclosed the reaction for producing low-carbon alkenes from methanol or dimethyl ether using a ZSM-5 molecular sieve catalyst modified by phosphorus and lanthanum in U.S. Pat. No. 5,367,100. Following this reaction, the total selectivity for ethylene and propylene was only about 65 wt % and the total selectivity for ethylene, propylene and butylenes was more than 85 wt % in the gas-phase product.
In 1984, a series of novel silicoaluminophosphate molecular sieves (SAPO-n) were developed in the Union Carbide Corporation (UCC) (U.S. Pat. No. 4,440,871). Thereinto, SAPO-34 molecular sieve exhibited an excellent catalytic performance in MTO reaction because of its proper acidity and pore structure. Replacing the ZSM-5 zeolite molecular sieve, it became the active component in the new generation of MTO catalysts. Because the pores in SAPO-34 have smaller sizes and elliptical cage structure, and coking deactivation occurs easily in methanol conversion, the MTO catalyst which utilizes SAPO-34 molecular sieve as its active component needs to be made into a microspherical fluidized catalyst and is applied to fluidization reaction process. Loss of catalysts cannot be avoided in a fluidization reaction, due to the frequent regeneration and abrasion of catalysts. As a result, the production cost of the MTO process is increased.
In order to overcome the shortages found in the fluidized bed technology for producing alkenes from methanol, development of anti-coking catalysts and fixed bed technology (involving less catalyst abrasion) is still an important research direction in this field. As known from the above prior art, while on the catalyst which utilizes ZSM-5 zeolite molecular sieve as its active component, one of the major reasons for causing a lower selectivity for ethylene and propylene in the products is that a great amount of arene products are generated in methanol conversion. Therefore, the routes for improving the selectivity for ethylene and propylene in the products include: (1) further converting the obtained arenes into alkenes; or (2) recycling the obtained arenes to facilitate the production of alkenes and suppress the generation of arene products.